The invention relates to manure digestion to produce methane gas, which may be used directly for process heat or to fuel an engine/generator to produce electrical power.
The extraction of energy from wastes using anaerobic digestion is old, and the general technology is known. It has been used to treat sewage sludge, and often the resulting biogas has been used to maintain the digester temperature, to run internal combustion engines, etc. Many small units were used in France and Algeria after World War II, and in India and China, many homes and farms have used biogas for everyday activities such as cooking, hot water heating, etc.
Anaerobic digestion is a microbial process involving biochemical decomposition of organic material such as animal manures in the absence of oxygen. It is a two-stage decomposition process. A specific group of microorganisms convert the complex organic matter present in the farm animal manure into methane and other gases. The first stage of this biochemical conversion process produces simple organic acids by acid-forming bacteria. In the second stage, mathane-forming bacteria consume and break down the acids into methane and carbon dioxide gases. Only a properly designed digester can claim to achieve the balance of the two groups of bacteria so that the methane-formers use all the acids produced by the acid-formers. This balancing can only be achieved by controlling the nutrient loading rate, retention time, mixing and temperature.
There are various environmental parameters such as pH, volatile acid concentration, temperature, nutrient availability, heating and heat balance that influence the ability of the bacteria to produce optimum quantities of biogas. Two distinct temperature ranges offer optimum conditions for bacterial growth in the anaerobic digestion process.
The first is mesophilic range, (85.degree. F.-110.degree. F.) and the second is the thermophilic range, (110.degree. F.-150.degree. F.). Although higher biogas production has been reported from digesters operating in the thermophilic range, the active bacteria are more sensitive to environmental conditions than mesophilic bacteria and therefore commercial scale digester control operation becomes too critical. Moreover, considerable energy input is needed to maintain the digester in the thermophilic range.
The mesophilic anaerobic digester of the present invention is structured to maintain the optimum balance between the acid and methane former bacteria and thereby to produce the maximum biogas from the digestion of the animal manures.
Various systems have been suggested for digesting agricultural manure with microbes, collecting methane gas produced thereby, and using the gas to fuel an engine which drives a generator to produce electrical power. Some of these systems have employed expandable covers on manure digester tanks or in-ground troughs, to accommodate variations in volume of gas being produced. See, for example, U.S. Pat. No. 4,274,838. Some have used various means for maintaining desired temperature in the digester tank, for optimum gas production. Often the prior systems were on a batch process, with new manure slurry added on a periodic batch-by-batch basis. Digester tanks of various configurations have been suggested. The waste heat from a methane gas-fueled engine fed by the digestion process has sometimes been used for heating the manure slurry in the digester.
However, none of the prior art systems contemplated or suggested the efficient combination of features of the present invention described below, for optimizing power output in a manure digesting/power generating system which is versatile, reliable and relatively simple in construction and arrangement.